The present invention relates to a staged process and apparatus for producing glass or the like of the type disclosed in U.S. Patent application Ser. No. 91,178 filed on Aug. 31, 1987. Although specifically applicable to production of vitreous glass products such as flat glass, fiber glass, container glass, or sodium silicate glass, the invention is also applicable to similar products that may not be considered "glass" under strict definitions. It should be understood that the term "glass" is used herein in the broader sense to include glass-like products. On the other hand, because of the higher standards for optical quality of flat glass, the improvements in refining achieved by the present invention are particularly significant to the production of flat glass.
In U.S. Pat. No. 4,381,934 to Kunkle et al. there is disclosed a process for performing the initial step of the melting process, rendering pulverulent batch materials to a liquefied, partially melted state. This process requires that the melting process be completed by a subsequent process stage for most glass products. Refining of the liquefied material would be a typical task of the subsequent process stage. In the Kunkle et al. patent, it is disclosed that the refining may be carried out by feeding the liquefied material to a conventional tank-type melting furnace. In order to optimize the economies of construction and operation of such a staged melting and refining operation, it is desirable to carry out the refining in as efficient a manner as possible, thereby minimizing the size of the refining apparatus and the energy consumed therein.
In the melting of glass, substantial quantities of gas are produced as a result of decomposition of batch materials. Other gases are physically entrained by the batch from combustion heat sources. Most of the gas escapes during the initial phase of melting, but some becomes entrapped in the melt. A primary objective of refining is to provide sufficient time and temperature conditions for substantial portions of these entrapped gases to be eliminated from the melt. Because elevated temperatures expedite the rise and escape of gaseous inclusions, the highest temperatures of the melting process are typically provided in the refining zone. Additionally, thermal conditions are conventionally controlled in the refining zone to maintain recirculating flows of the molten glass in order to provide adequate residence time and to assure that the throughput stream passes through the region at high temperatures, where gases are released into the space above the melt, and that unrefined portions of the melt are directed away from the throughput stream. Additionally, the refining stage may be employed to assure dissolution of any remaining solid particles of the batch. Furthermore, the recirculation established during refining can be useful in homogenizing the melt. It would be desirable to optimize the achievement of at least some, and preferably all, of these objectives of refining when coupled to a discrete liquefying stage as in U.S. Pat. No. 4,381,934.
A difficulty arises from the fact that material discharged from a liquefying stage is only partially melted, typically being in a substantially foamy condition with unmelted solid particles. When such material is passed to a pool of molten glass in a refining furnace, the material tends to stratify near the surface of the pool. This stratified material has been found to not respond to the recirculating flows within the main portion of the pool that assure adequate residence time and temperature exposure to accomplish the refining step. Accordingly, discharging material from a liquefying stage directly to a recirculating refining furnace as shown in U.S. Pat. No. 4,381,934 has been found to yield inadequate refining.
Another problem is that maintaining the desired convection flow patterns in the refiner is more difficult when the material entering the refiner is liquefied. This is because in a conventional tank type melting and refining furnace the unmelted batch materials fed onto the molten pool serve as a heat sink at one end of the pool, thereby creating a downward flow in that region which contributes to sustaining a strong circulation pattern. Such an effect is not present to as great an extent when the batch materials are liquefied at a separate location. When there is insufficient recirculation in the refiner, the probability increases that a portion of the material will pass quickly to the outgoing product stream, thereby contaminating the product with inadequately refined glass.
In the aforesaid U.S. patent application, an arrangement is proposed whereby glass batch material or the like is liquefied and refined in discrete, physically separated stages, but instead of passing the liquefied material directly to the refining stage, it passes through an intermediate stage where it is readied for entry into the refiner. By providing an intermediate receiving vessel, the stratified foam layer can be contained separately from the refiner, the temperature of the material can be increased so as to be more compatible with the desired convection flow patterns in the refiner, and undissolved sand grains and the like may be provided with sufficient residence time to substantially completely dissolve before entering the refiner.
A preferred embodiment for effecting the intermediate processing of liquefied material being fed to the refiner is an elongated, narrow channel. Typically, the channel has length and width considerably less than that of the refiner. Advantageously, a plurality of liquefying stages may feed a single refiner, in which case each is preferably provided with a channel connecting it with the refiner. Since the primary function of the channel is to permit the liquefied material to be heated to a higher temperature, substantial volume in the channel is not necessary.
The use of a relatively compact channel as the intermediate stage has advantages but also is accompanied by problems. One problem is that the foamy, low density, partially melted material can be difficult to retain in the small vessel. Event when a surface barrier is employed, the short distances involved in the vessel sometimes permit the low density material to be drawn into the stream flowing to the refiner. Extending a barrier farther below the surface of the melt may provide better blockage of surface entrainment but leads to undesirably effects in a system of the type to which the present invention relates. A barrier submerged deeply into the melt leaves less of an opening below for passage of material, thereby resulting in higher velocities of flow through the opening, particularly when the vessel is a compact channel. High velocity flow under a barrier made of ceramic refractories can produce accelerated erosion of the barrier and undesirable contamination of the molten material. Employing a non-contaminating material such as platinum is prohibitively costly for such an application. Less costly metal such as stainless steel could serve as a barrier provided that it is cooled, but the amount of cooled surface area entailed by a cooled barrier for this situation can result in unduly great heat extraction from the melt. The cooling is not only at odds with the purpose of the channel to heat the molten material, but also tends to create undesirable flow patterns in the melt retained in the channel. A cooled barrier produces a downward current at the outlet end of the channel which can entrain low density surface material into the flow stream leaving the channel. Also, cooling increases temperature differences between regions within the melt, thereby serving as a greater driving force for circulation, which is undesirable in this environment where an objective is to prolong the residence time in the channel of each increment of melt.